Conventionally, for an image taking process using an X-ray Computed Tomography (CT) apparatus, it is possible to perform a metal artifact reducing process to reduce artifacts caused by metal contained in the body of the subject. For example, the metal artifact reducing process is able to reduce artifacts caused by intracorporeal metal such as intraoral metal, intravascular metal, metal fixtures used for securing bones, other medical metal pieces, unwanted metallic intracorporeal matters, and the like and to improve the level of precision of diagnosing processes performed by using CT images. In the present example, the intraoral metal refers to metal that is placed to remain in the mouth for a dental treatment or the like, and examples also include dental implants. The intravascular metal refers to stents, coils, filters, and artificial valves, for example. Examples of the metal fixtures used for securing bones include plates, screws, wires, and the like. Further, examples of other medical metal pieces include clips, staples, implants, artificial joints, artificial bone heads, and pacemakers. Examples of the unwanted metallic intracorporeal matters include metal pieces and bullets.